Persistence of Cav1.3 Ca2+ Channels in Mature Outer Hair Cells Supports Outer Hair Cell Afferent Signaling

Abstract

Outer hair cells (OHCs) are innervated by type II afferent fibers of as yet unknown function. It is still a matter of debate whether OHCs perform exocytosis. If so, they would require presynaptic Ca2+ channels at their basal poles where the type II fibers make contacts. Here we show that L-type Ca2+ channel currents (charge carrier, 10 mM Ba2+) present in neonatal OHCs [postnatal day 1 (P1) to P7] decreased from approximately 170 to approximately 50 pA at approximately the onset of hearing. Ba2+ currents could hardly be measured in mature mouse OHCs because of their high fragility, whereas in the rat, the average Ba2+ current amplitude of apical OHCs was 58 +/- 9 pA (n = 20, P19-P30) compared with that of the inner hair cells (IHCs) of 181 +/- 50 pA (n = 24, P17-P30). Properties of Ba2+ currents of mature OHCs resembled those of neonatal OHCs. One exception was the voltage dependence of activation that shifted between birth and P12 by +9 mV toward positive voltages in OHCs, whereas it remained constant in the IHCs. Ca(v)1.3-specific mRNA was detected in mature OHCs using cell-specific reverse transcription (RT)-PCR and in situ hybridization. Ca(v)1.3 protein was stained exclusively at the base of mature OHCs, in colocalization with the ribbon synapse protein CtBP2 (C-terminal binding protein 2)/RIBEYE. When current sizes were normalized to the estimated number of afferent fibers or presynaptic ribbons, comparable values for IHCs and OHCs were obtained, a finding that together with the colocalization of Ca(v)1.3 and CtBP2/RIBEYE protein strongly suggests a role for Ca(v)1.3 channels in exocytosis of mature OHCs.

Ephrin-A5/EphA4 signalling controls specific afferent targeting to cochlear hair cells

“…During the first postnatal week, type I SGNs extend transient collateral projections to OHCs 5 and form synapses that express glutamate receptor subtypes 48 and trigger Ca 2 þ -and otoferlin-dependent glutamate release, similar to what occurs in mature IHCs 49 . In adult cochlea, ribbons are believed to support afferent signalling from OHCs 50 . Concurrently, these findings suggest the idea that, in either ephrin-A5 À / À or EphA4 forward signalling-deficient mice, some definitive type I-OHC ribbon-containing synapses could enhance the afferent neurotransmission from this region.…”

Reciprocal Synapses Between Outer Hair Cells and their Afferent Terminals: Evidence for a Local Neural Network in the Mammalian Cochlea

“…Immunohistochemistry shows voltage-gated calcium channels (Cav1.3) at the basal pole of adult OHCs, and voltage-dependent calcium currents have been measured in isolated OHCs at least as late as P30 (Knirsch et al 2007). Immature OHCs (at P3) show capacitance changes elicited by voltage steps, consistent with vesicle release (Beurg et al 2008).…”

The Structural and Functional Differentiation of Hair Cells in a Lizard's Basilar Papilla Suggests an Operational Principle of Amniote Cochleas

“…For example, mammalian inner hair cells lack the prominent nonlinear capacitance associated with the membrane-based electromotility of outer hair cells (Dallos, 1992;Nobili et al, 1998;Dallos et al, 2006). Another, less dramatic difference occurs in the level of expression of certain ion conductances: the macroscopic currents through Ca 2ϩ and Ca 2ϩ -activated K ϩ channels in mammalian inner hair cells and avian tall hair cells exceed those in the corresponding cells of the other type (Kros and Crawford, 1990;Martinez-Dunst et al, 1997;Kros et al, 1998;Skinner et al, 2003;Pyott et al, 2004;Knirsch et al, 2007).…”